I have a low pressure heating system (gas boiler with hot water cylinder, vents, etc) and even though I have switched off the central heating on the control panel for the summer, the radiators heat up when the hot water system is activated.

My immediate suspicion is the the solenoid switch that directs the flow of hot water to each system. Even if I set it to "manual", it still heats up the radiators.

remove the actuator head and make sure the valve spindle moves freely, then try the actuator making sure it moves round when water, heating or both selected, sometimes the actuator head gets chewed where it sits on the valve, so even setting the actuator to manual the valve may not be turning.